This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-036003, filed Feb. 15, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a manufacturing method for a hollow stabilizer used in a vehicle such as an automobile.
Conventionally, a stabilizer is used as means for enhancing the roll stiffness of a vehicle. A hollow stabilizer using a pipe is known as a version that has developed in order to reduce the vehicle weight. FIG. 6 shows manufacturing processes for a conventional hollow stabilizer. In general, a seam-welded steel pipe of low carbon steel (carbon content: 0.2 to 0.3%) is used for the conventional hollow stabilizer. After the seam-welded pipe is formed into a desired stabilizer shape in a cold state, it is heated and subjected to heat treatment such as quenching and tempering for thermally refined, and is further shot- peened.
The seam-welded pipe is manufactured in a manner such that a hot-rolled and hooped sheet is formed into a pipe with a circular cross section by the conven- tional roll forming method as seams along its axial direction are welded. Accordingly, the wavelength thickness ratio (wall thickness/outside diameter) of pipes that can be manufactured by the roll forming method cannot be adjusted to a very high value. Thus, it is hard for normal equipment to manufacture seam-welded pipes with high wall thickness ratios exceeding 0.15-0.17.
Heretofore, therefore, a hollow stabilizer that is based on a seam-welded pipe must inevitably use a thin-walled pipe with a relatively low wall thickness ratio. In view of lightness in weight, the thin-walled pipe is a material superior to a conventional stabilizer that is formed of a solid material (solid steel rod). The lower the wall thickness ratio (or the thinner the wall of the pipe), the higher the design stress of the stabilizer is, and therefore, the more the stress is restricted. The design stress is a reference stress that is based on a regular stabilizer shape and a fixed spring constant. Thus, a seam-welded pipe can be used if there is room for stress in designing the stabilizer. The use of the seam-welded pipe, compared to the solid stabilizer, can achieve a reduction of 50% or more in weight.
In the case of some high-stress hollow stabilizers, however, the design criterion cannot be cleared even with use of seam-welded pipes with the highest available wall thickness ratio of 0.15 to 0.17 (rate of weight reduction: 40% to 35%). In this case, solid rods should be employed inevitably. Despite the advantage of their lightness in weight, therefore, the hollow stabilizers presently account for a share of only scores of percent in the field of stabilizer production.
Accordingly, the object of the present invention is to provide a manufacturing method for a hollow stabilizer, whereby the aforementioned drawbacks attributable to the use of seam-welded pipes for stabilizers can be eliminated, and a plan for putting hollow stabilizers to practical use can be expedited.
In order to achieve the above object, a manufacturing method according to the present invention comprises a primary pipe manufacturing process for subjecting a seam-welded pipe to continuous hot reduction working, thereby obtaining a primary pipe, and winding the primary pipe in a coil, a cold drawing process for subjecting the primary pipe to cold drawing so that the rate of reduction in area is not lower than 30%, thereby obtaining a thick-walled pipe having a wall thickness ratio of 0.20 to 0.27 higher than that of the seam-welded pipe and tensile strength of 800 N/mm2 to 1,000 N/mm2 is obtained, and a bending process for bending the thick-walled pipe, obtained in the cold drawing process, into a desired stabilizer shape in a cold state.
According to the invention, the drawbacks of the conventional hollow stabilizers that are formed of seam-welded pipes can be eliminated even with use of a seam-welded pipe, and the manufactured stabilizer can be made lighter enough in weight than the conventional solid stabilizers. The hollow stabilizer manufactured according to the invention can enjoy a desired durability and obviate the necessity of heat treatment including quenching and tempering. Thus, the necessary manufacturing equipment and energy for the heat treatment can be saved, so that the manufacturing cost can be lowered.
The manufacturing method of the invention may further comprise an annealing process for subjecting the thick-walled pipe to stress relieving annealing at a temperature of 300xc2x0 C. to 450xc2x0 C., after the bending process, and a shot peening process for cooling the thick-walled pipe to room temperature and then shot-peening the same, after the annealing process. Alternatively, the method of the invention may further comprise an annealing process for subjecting the thick-walled pipe to stress relieving annealing at a temperature of 300xc2x0 C. to 450xc2x0 C., after the bending process, and a shot peening process for shot-peening the thick-walled pipe while the pipe is kept at a temperature of 300xc2x0 C. to 400xc2x0 C., after the annealing process. By carrying out the annealing process and the shot peening process, as well as by the aforementioned effect of the invention, the durability of the hollow stabilizer can be improved.
Alternatively, furthermore, the manufacturing method of the invention may further comprise an internal shot peening process for shot-peening the inner surface of the thick-walled pipe before carrying out the bending process. This internal shot peening process can further improve the durability of the hollow stabilizer of the invention.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.